1. Field of the Invention
The present invention relates to an object proximity detector. In particular, the present invention relates to an object proximity detector or position detector with reduced interference of external electrical and magnetic wave.
2. Description of the Related Art
The capacitive sensing technology is popular in the design of object proximity detector and position detector. The capacitances of capacitor and sensor plate used in the sensor circuit are very small. Small capacitance means high impedance and bad noise immunity. But in many applications, immunity from the electromagnetic interference is very important. There is a variety of technologies to design object proximity detector and object position detector. One of the technologies is to feed an AC signal to an amplifier through a capacitor and a sensor plate and to measure the variation of amplitude of the AC signal at the output of the amplifier. The technology is used in U.S. Pat. No. 5,374,787, U.S. Pat. No. 5,495,077, U.S. Pat. No. 5,841,078, to Robert J. Miller et al., U.S. Pat. No. 5,914,465, U.S. Pat. No. 6,239,389B1, to Timothy P. Allen et al., U.S. Pat. No. 6,028,271, U.S. Pat. No. 6,610,936B2, to David W. Gillespie et al. In these systems, the immunity of electromagnetic interference from sensor plate is seldom mentioned. The technology used in U.S. Pat. No. 6,452,514B1 and U.S. Pat. No. 6,466,036 B1, to Harald Philipp is a charge transfer circuit. In this circuit, an AC voltage source is applied to one plate of the sensor and fed into a signal processor through the other plate of the sensor. The charge transfer circuit is used to reduce the influence of the noise. The signal processor consists of charge transfer circuit, integrator and voltage measurement circuit. Besides the analog circuits mentioned above, a lot of high speed switches are used. The clock feed-through caused by the parasitic capacitance of the high speed analog switches will cause another interference of noise and distortion of the signal. One of the capacitive object proximity detector and position detector was designed by the inventor as described in the U.S. patent (U.S. Pat. No. 7,023,221, B1), European patent (EP 1791260 B1) and Taiwan, R.O.C. patent (I 259908). The basic circuit of the invention is illustrated in FIG. 1. FIG. 1 is an oscillator circuit of an object proximity detector with a single pair of sensor plates. This circuit consists of three inverters, 101, 102, 103, a resistor 104, two capacitors, 106, 107 and a pair of sensor plates (105) with capacitance Cs. The frequency of the oscillator is determined by the resistance of the resistor 104 and capacitance of capacitor 106, 107 and sensor plates 105. The proximity of an object to the sensor plates 105 will change the capacitance of the sensor plates, and the frequency of the oscillator. The proximity of an object to the sensor plates will be detected by the variation of the frequency of the oscillator. There are two advantages of the object proximity detector. Independence of the sensitivity of the object proximity detector with process parameters variation is one of the advantages. The other advantage is the high sensitivity of the object proximity detector.
The sensor plates of FIG. 1 are in the feedback loop of the oscillator and outside of the integrated circuit. The capacitance of the sensor plates will affect the frequency of the oscillator significantly. Besides, if the area of the sensor plates is big enough, the external electrical and magnetic interference will affect the performance of the oscillator significantly. FIG. 2 is an oscillator of object proximity detector with two pairs of sensor plates which can detect the proximity of an object at two different positions. Both pairs of sensor plates 205 and 206 are at the feedback loop of the oscillator and in series with transmission gates 208 and 209 respectively. Only one of the transmission gates is conducting at one time. In some application, it is necessary to design a system with different area of sensor plates. Different area of sensor plates causes different capacitance. And the frequency of the oscillator will be different when switch to different transmission gate. If the difference of the frequency is big enough, it is difficult to set the sensitivity of the proximity detector in a system. Therefore it is one of the objects of the invention to design a proximity detector to reduce the dependence of the frequency on the area of the sensor plates. In the circuit of the prior art, as shown in FIG. 1, there are two connecting wires 108 and 109 connecting the sensor plates 105 to the oscillator circuit 100. In FIG. 1, if the connecting wires are very long, the capacitance between the two wires has considerable effect on the frequency of the oscillator. Besides, if the two connecting wires are flexible, the variation of the distance between the two wires will cause the variation of the capacitance between the two wires. Variation of the capacitance will cause variation of the frequency and error detection of the object proximity detector. Therefore it is another object of the invention is to design a proximity detector to reduce the effect of the capacitance between the two conducting wires on the frequency of the oscillator. The other object of the invention is to design a proximity detector to reduce the influence of the external electrical and magnetic interference. In the design of the circuit of invention, we keep the compensating capacitor which is also used in the circuit of the prior invention, and the dependence of oscillator frequency on the process parameters can be kept to a minimum. Besides, the sensor plate and two capacitors which connecting the sensor plates are also in the feedback loop of the oscillator, and high sensitivity of the object proximity can be kept. In summary, in designing an object proximity detector and object position detector, we want not only to keep the advantages of the prior art of invention but also to improve the immunity of electromagnetic interference, the independence of sensitivity on sensor plate area and to allow the flexibility of conducting wire of the sensor plate.